I have an H190 with a 4.38 LSD now that id consider swapping for a 4.62 if that helps you any. I'd have to run the numbers on a 4.88 gear to see its to low for what you are doing.

EDIT: I ran the numbers for a 4.88 final drive with a 1-1 5th gear your top speed at 8000 rooms with a 185/60-13 radial would be 106 and with 205/60-13 would be 111.

111 is what I'm doing at the top of the hill at Spring Mtn. I sometimes run the motor to 8200 which your A12 would easily do so you could probably get as much as 115mph out of it.